Nonvolatile memory elements are used in systems in which persistent storage is required. For example, digital cameras use nonvolatile memory cards to store images and digital music players use nonvolatile memory to store audio data. Nonvolatile memory is also used to persistently store data in computer environments.
Nonvolatile memory is often formed using electrically-erasable programmable read only memory (EPROM) technology. This type of nonvolatile memory contains floating gate transistors that can be selectively programmed or erased by application of suitable voltages to their terminals.
As fabrication techniques improve, it is becoming possible to fabricate nonvolatile memory elements with increasingly small dimensions. However, as device dimensions shrink, scaling issues are posing challenges for traditional nonvolatile memory technology. This has lead to the investigation of alternative nonvolatile memory technologies, including resistive switching nonvolatile memory.
Resistive switching nonvolatile memory is formed using memory elements that have two or more stable states with different resistances. Bistable memory has two stable states. A bistable memory element can be placed in a high resistance state or a low resistance state by application of voltage pulses. Nondestructive read operations can be performed to ascertain the value of a data bit that is stored in a memory cell.
Nonvolatile memory elements can be formed using metal oxides. Resistive switching based on nickel oxide switching elements and noble metal electrodes such as platinum electrodes has been demonstrated.
In a typical scenario, a stack of resistive switching oxide and electrode layers is deposited using physical vapor deposition (PVD) (sputtering). Dry etching is then used to pattern the deposited layers. Heat may be applied to the deposited structures during this type fabrication process.
Arrangements such as these may be satisfactory, but can give rise to processing challenges. For example, multilayer materials that undergo high temperature treatments may lose interfacial integrity due to interdiffusion or stress-induced delamination.
Moreover, the use of conventional rapid thermal oxidation processes or other such post-processing oxidation techniques to form resistive switching metal oxide films may lead to undesirable oxidation of contact pads or other device features.
It would therefore be desirable to be able to provide improved techniques for forming resistive switching structures using thermal processes.